Ink jet printing mechanisms use pens that shoot droplets of colorant onto a printable surface to generate an image. Such mechanisms may be used in a wide variety of applications, including computer printers, plotters, copiers, and facsimile machines. For convenience, the concepts of the invention are discussed in the context of a printer.
Color ink jet printers generally use cyan, magenta, and yellow inks to generate a full range of colors in a printed image. Black ink is used to generate portions of the output containing text and other black images, and to enhance the appearance of color image tones. While equal proportions of the three color inks may be used to create gray or black tones, it is sometimes preferable to use black ink to provide improved output appearance, reduced printing cost, and increased printing speed. In existing designs, the color cartridge typically has three short elongated arrays of nozzles, one for each color, while the black cartridge has a single, much longer high resolution array, permitting faster printing rates when no color is present.
Glossy material is often used as a printer media. Plastic film may be used for overhead projector slide transparencies, and plastic coated paper may be used to generate optimum color image quality for images having a nearly photographic appearance. Other films and foils may be used for various applications. Glossy media normally has a surface that admits little or no penetration of liquid inks. The surface tension of may inks is normally controlled to avoid unwanted "beading up" on the
When printing color images that include black portions, the different inks may interact with each other to generate unwanted image defects.
Color and black inks are often selected to have different but compatible characteristics that avoid most undesirable interactions. These characteristics include rate of penetration, density of pigmentation, dry time, and water fastness. Inks may be formulated to avoid halo, but these formulations may compromise other important ink characteristics. Existing printing techniques to minimize halo include printing with additional overlapping multiple passes of partial density, heating of the print media, and/or pausing to facilitate drying between printing passes. These approaches are more expensive, or undesirably reduce printing speed.
The apparatus and method disclosed herein overcomes the above disadvantages by providing a method of processing print data by scanning the print data to determine a black density level and a color density level. A fortification function in which color density varies with respect to black density is applied, and if the color density level is less than a predetermined value based on the fortification function at the given black density level, the color density level is increased to the predetermined value. The method of processing may be performed in conjunction with the process of printing by an ink jet printer, and the fortification function may provide a color density of zero below a first threshold of black density, an increasing density up to a second threshold, and a constant density above the second threshold of black density.